The present invention relates to a combined roughing and finishing circuit in which the finishing circuit is enabled when the machining pulse "on" time has been shortened to a point that requires fine finishing. In electrical discharge machining, there is provided a tool electrode and an electrically conductive workpiece which are separated by a dielectric filled gap. A main power supply and an electronic switch are connected by turning the electronic switch on and off with predetermined on and off times thus passing erosive electrical discharges across the gap. It has been found that a servo means must be included to provide relative movement between the electrode and workpiece as material is being removed. Because of stray capacitance in the power lead cable, the voltage at the electrode cannot drop to zero immediately after the electronic switch is turned off. This situation is accentuated during the fine finish operation in which relatively short on and off times are used. In order to improve the surface finish in a finishing operation, it is necessary to lower the machining energy in each pulse by altering the voltage waveform. Reference is made to, U.S. Pat. Application No. 834,375 filed on 02/28/86 for "FINE FINISH CIRCUIT FOR ELECTRICAL DISCHARGE MACHINING APPARATUS" of common ownership with the present application.
The present invention relates to a fine finish circuit that may be connected in the circuit across the gap at the appropriate time as the finishing operation is being undertaken. The finishing circuit includes an electronic switch operably connected across the EDM gap at an appropriate time in the machining operation. When the main electronic switch is triggered on a pulse occurrs across the gap and the second electronic switch is then triggered on to cause an abrupt drop in gap voltage and an off time and therefor diminishing the current in the gap during the finishing operation. The present invention includes a system for determining the appropriate time for the enabling of the finishing circuit .